1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) device, and more particularly, to an LCD device including a touch screen which reduces the influence of noise applied to the touch screen by driving of a liquid crystal panel and thus leads to enhancement of the touch sensing performance.
2. Discussion of the Related Art
With the advance of various portable electronic devices such as mobile communication terminals and notebook computers, the demands of Flat Panel Display (FPD) devices applicable to the portable electronic devices are increasing.
Liquid Crystal Display (LCD) devices, Plasma Display Panels (PDPs), Field Emission Display (FED) devices, and Light Emitting Diode (LED) display devices have been developed as FPD devices.
In such FPD devices, the application fields of LCD devices are being expanded because the LCD devices are easily manufactured and have drivability of drivers, low power consumption, thin thickness, high image quality and a large screen.
Touch screens, which replace the existing input devices such as mouses or keyboards and enable a user to directly input information by finger and pen, are being applied as the input devices of LCD devices.
Touch screens can be easily manipulated by all users, and thus, the applications of the touch screens are being expanded.
Such touch screens are categorized into the following types according to structures thereof. An in-cell type where a touch screen is built in a cell of a liquid crystal panel. An on-cell type where a touch screen is disposed at an upper portion of a liquid crystal panel. An add-on type where a touch screen is separately coupled to an upper portion of an LCD device.
FIG. 1 is a view illustrating a related art LCD device with an on-cell type touch screen applied thereto. FIG. 2 is a view illustrating the structure of the touch screen of FIG. 1.
Referring to FIGS. 1 and 2, the related art LCD device with the on-cell type touch screen applied thereto includes a backlight unit 10, a liquid crystal panel 20, a lower polarizer 30, an upper polarizer 40, a touch screen 50, and a protective film 60 (or a protective glass).
The liquid crystal panel 20 includes a lower substrate 22 and an upper substrate 24 that are coupled to each other with a liquid crystal layer (not shown) therebetween.
A plurality of pixels are formed in a matrix type, at the lower substrate 22.
Red, green, and blue color filters are formed in a matrix type, at the upper substrate 24.
The lower polarizer 30 is disposed at a lower portion of the liquid crystal panel 20, and the upper polarizer 40 is disposed at an upper portion of the liquid crystal panel 20.
Herein, the backlight unit 10 includes a light source 12 that emits light, a light guide panel 14, and a plurality of optical sheets 16.
The light guide panel 14 guides light, supplied from the light source 12, toward the liquid crystal panel 20. The optical sheets 16 enhance the efficiency of light that is outputted from the light guide panel 14 and is incident on the liquid crystal panel 20.
The touch screen 50, as illustrated in FIG. 2, includes a plurality of first touch electrodes 52 that are formed in an X-axis direction, and a plurality of second touch electrodes 54 that are formed in a Y-axis direction.
The first touch electrodes 52 and the second touch electrodes 54 are formed on the upper substrate 24 of the liquid crystal panel 20. A plurality of insulation layers are respectively disposed in a plurality of regions where the first touch electrodes 52 intersect the second touch electrodes 54, and form a bridge. The first and second touch electrodes 52 and are insulated from each other by the bridge.
A signal (voltage) for detecting a touch is applied to the first touch electrode 52.
When a user touches a specific point of a display screen (touch screen panel) by finger or pen, a capacitance between the first and second touch electrodes 52 and 54 is changed by the user's touch.
A touch sensing driver, which is prepared separately, senses the change of the capacitance between the first and second touch electrodes 52 and 54, thereby sensing a touch point.
In the related art LCD device including the on-cell type touch screen, the first and second touch electrodes 52 and 54 for sensing a touch are formed of indium tin oxide (ITO) on the upper substrate 24 with the color filters formed therein and thus configure the touch screen.
The liquid crystal panel 20 adjusts an amount of transmitted light by pixel unit with electric fields that are generated in the pixels, and thus displays a color image. At this point, a current in each pixel is changed by driving of a screen, causing noise such as an electromagnetic wave.
The first touch electrode 52 (X-axis touch electrode) and second touch electrode 54 (Y-axis touch electrode) of the touch screen 50 are formed on the upper substrate 24, and consequently affected by noise that occurs in the liquid crystal panel 20.
Particularly, a capacitive type touch screen is vulnerable to the noise. In the on-cell type touch screen 50, since the two touch electrodes 52 and 54 are formed on the upper substrate 24 close to the lower substrate 22 with thin film transistors formed therein, the on-cell type touch screen 50 is more affected by noise.
As described above, due to characteristic of the stack-up structure of the touch screen 50 formed on the upper substrate 24 of the liquid crystal panel 20, the touch screen 50 is vulnerable to noise that is caused by driving of the liquid crystal panel 20, and consequently touch sensing performance is degraded.
Recently, LCD devices including a touch screen are being increasingly applied to mobile devices such as portable phones and smart phones. Design, communication quality, and touch sensing performance are becoming factors important for purchasing mobile devices.
Therefore, all manufacturers are researching technology for enhancing the touch sensing performance of the on-cell type touch screen, but technology for innovatively enhancing the touch sensing performance is not developed up to date.
Moreover, to decrease the influence of noise due to driving of the liquid crystal panel 20, the design of the touch screen 50 is required to be changed, but the related art has limitations in changing the design of the touch screen 50 for reducing the influence of noise.
Furthermore, a touch sensing driver robust to noise is required to be applied to touch screens. However, the touch sensing driver robust to noise is restrictively applied to some products, and the cost increases in applying the touch sensing driver.